The subject matter disclosed herein relates to fuel nozzles and more particularly to a nozzle which shuts off fuel feed if a flame enters the nozzle.
The majority of pre-mix fuel nozzle builds are designed to pre-mix natural gas fuel. Today there is an emphasis on designing and building fuel nozzles that burn a hydrogen fuel. Hydrogen fuel is much more reactive, and thus, has a much higher flame speed. When designing fuel nozzles for pre-mixed combustion systems, the air and fuel are introduced upstream of where the combustion process takes place. Generally, fuel nozzles are designed to flow air through them at a rate that is faster than the flame can propagate upstream. When the fuel used is hydrogen, it is much more difficult to keep the flame out of the fuel nozzle. If the flame “flashes back” into the pre-mixer for any length of time it will destroy the fuel nozzle, since the flame temperature is almost always higher than the melting temperatures of the nozzle parts. If a nozzle cannot reliably keep the flame out of the fuel nozzle, other alternatives must be considered.
Flashback damage has historically been detected using NOx emission and exhaust temperature spreads as indicators. When a flashback occurs, NOx increases and exhaust temperature spreads often, but not always, increase. The NOx increase is typically proportional to the severity of the flashback. Further, the exhaust temperature spread change can vary, either decreasing or increasing, depending upon the state of the combustors, which suffer flashback, prior to the flashback event. The unpredictable behavior of exhaust temperature spreads, coupled with the emissions data scatter, has made it difficult to determine whether or not a flashback has occurred using NOx and exhaust spread indicators. Therefore, methods which rely on changes in NOx and exhaust profile over sequential instants of time to determine if a flashback has occurred are ineffective.
Other methods for detecting flashback events in gas turbines include periodic reference point checks to determine whether or not flashback damage has occurred. The method relies on the repeatability of exhaust profile and NOx as functions of turbine conditions. In combination with experience-based limits, changes in these values are used to determine if a flashback has occurred, even days later. This does not help determine a flashback event at the instant it occurs.
Normally, it would be advantageous for a flash back event to be actively extinguished when it occurs. This requires first sensing the flashback event and, when detected, turning off a valve and then re-starting the fuel flow after the flame goes out. As discussed above, the process of first sensing the flashback event is an unreliable or slow process. Even were it possible to instantly detect flashback, it is still necessary to turn off fuel flow to the nozzle. If the flashback event is not corrected in a very short period of time, or if the flashback causes a flame holding event within the nozzle, the nozzle can be irreparably damaged or destroyed.
The cost of adding flashback sensing equipment, control equipment and control valves to each nozzle is expensive. In addition it is not practical to implement a control system on many individual injectors, which it is expected will be required in order to consistently burn hydrogen rich fuels. If these facts are coupled with the inability to accurately and quickly sense a flashback event, it is clear that another alternative is required.